Create angular .ts client code generator from openapi/swagger.json file (#783)

Based on https://github.com/heroiclabs/nakama-js/tree/master/openapi-gen:

adjusted to angular projects, mirroring the previous protoc generator template
added fixes and new schema elements
generates missing interfaces from POST+PUT requests that use path and body params (as <function_name+"Request">)

Differences to previous generated .ts client:

already url encodes all non-path parameters
accepts prefixes to remove as a command line flag, because swagger adds the name of imports/the file itself as prefixes to types and functions
map types are generated as Map<>, not as type arrays
array types are generated as Array<>, not as Type[]
swagger does not reference the enum type on function arguments, so enums need to be passed as strings